During the development of the nervous system, inexcitable precursor cells of the embryonic neural plate both extend processes to form synapses onto target cells, and acquire the complement of ion channels that determine adult excitability within the central nervous system. The proposed experiments are designed to 1) study the modifications of ion channels in neural membranes that drive developmental changes in the waveforms and ionic dependences of embryonic action potentials that culminate in the adult phenotype, and 2) examine the spatial segregation of ion channels within neuron somata, and between somata and growth cones, in order to study the role of ion fluxes, particularly Ca fluxes, in neural differentiation and the modulation of growth cone activity. I propose to carry out these investigations using a primary culture system involving the spinal region of the amphibian neural plate; certain of these precursor cells when grown in culture differentiate into neurons and innervate target cells. This system provides the advantages of relatively appropriate devlopment, and physical access to all areas of the cell membrane surface. The activities of Na, Ca and various K channels will be assayed using patch electrode voltage clamp techniques to record from whole soma and growth cone membranes, and from restricted portions of membrane. These studies will give a better understanding of the mechanisms by which the ionic currents of neurons are shaped during development, and of the role of the neural membrane in the regulation of the activities of growth cones.